Power over LAN™ is a new technology that enables DC power to be supplied to Ethernet data terminals over ordinary Category 5 cabling. This technology enables the terminals to receive their operating power over the same Ethernet local area network (LAN) that they use for data communication. It thus eliminates the need to connect each terminal to an AC power socket, and to provide each terminal with its own AC/DC power converter. Further aspects of this technology are described in PowerDsine Application Note 115, entitled “Power over LAN™: Building Power Ready Devices” (PowerDsine Ltd., Hod Hasharon, Israel), which is incorporated herein by reference. The LAN MAN Standards Committee of the IEEE Computer Society is developing specifications for Power over LAN systems, as described in IEEE Draft P802.3af/D3.0, entitled “Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI)” (IEEE Standards Department, Piscataway, N.J., 2001), which is also incorporated herein by reference.
A Power over LAN system comprises an Ethernet switch and a power hub, which serves as the DC power source, along with a number of terminals, which communicate via the switch and draw power from the hub. The system is typically connected in a star topology, with each terminal linked by a dedicated cable to the switch and hub. DC power is carried to the loads (i.e., the terminals) over the twisted pairs provided by Category 5 cabling that are not needed for Ethernet data communications. The power hub may be integrated with the switch, in what is known as an “end-span” configuration, or it may alternatively be located between the switch and the terminals, in a “mid-span” configuration. These alternative configurations are illustrated on pages 16 and 17 of the above-mentioned IEEE Draft.
To avoid possible equipment damage and safety hazards, the power hub must ensure that none of the loads that it serves draws current in excess of a maximum limit. The need for such current limiting is well known in the art of DC power supplies, and is not limited to the context of Power over LAN. The most common solution for this purpose is to place a sampling resistor and a variable-impedance current-limiting element in series with the load. The sampling resistor provides a differential voltage input to an integrating amplifier, which compares the input voltage to a preset reference. The amplifier output controls the impedance of the current-limiting element, which is typically a bipolar transistor or field effect transistor (FET) operating in its linear range. A digital integrator may be used in place of the integrating amplifier.
Conventional methods for current limiting of DC power supply output have a number of drawbacks, which are particularly problematic in the context of Power over LAN:                When current limiting is needed to prevent an overload, the current-limiting element in the power supply must dissipate substantial power. Current limiting may be needed not only in fault situations, but also under certain normal operating conditions. For example, when a terminal on the LAN is initially connected or turned on, it can create a virtual short circuit while its internal power capacitor charges up. The substantial power dissipation required of the current-limiting element creates thermal problems in the hub.        Because an integrating amplifier or digital integrator is used to control the current-limiting element, the response of the element to current changes is typically slow. On the other hand, reducing the time constant of the integrator can lead to instability and oscillations of the current-limiting impedance.        All the terminals that are powered by the hub must be protected individually against over current. The cost of the hub is therefore increased by the need to provide a separate sampling resistor, current-limiting element and control circuit for each connection in the LAN. Circuits based on pulse width modulation (PWM) are commonly used in applications such as power conversion and switching power supplies. Exemplary PWM circuits and systems for such purposes are described in U.S. Pat. Nos. 5,355,077, 5,969,515, and 6,268,716, whose disclosures are incorporated herein by reference.        